As described, for example, in Japanese Patent Application Publication No. H11-341576, a gooseneck condenser microphone has been used favorably in a conference facility such as an international conference hall from the viewpoint of its simple appearance and easy adjustment of angle and height.
In the gooseneck condenser microphone, a condenser microphone unit and an output module section are formed separately. The condenser microphone unit is supported on the output module section via a support pipe including a flexible pipe partially or wholly, and the condenser microphone unit can be brought close to the mouth of a speaker by means of the flexibility of the flexible pipe.
Usually, the condenser microphone unit incorporates a field effect transistor (FET) serving as an impedance converter. The output module section has a circuit board for outputting audio signals arranged in a cylindrical shield case, and the condenser microphone unit is connected to the circuit board via a dedicated microphone cable inserted in the support pipe.
The microphone cable includes a power line for supplying power to the condenser microphone unit, a signal line for sending audio signals generated from the impedance converter to the output module section, and a shield covering line for electrostatically shielding and grounding the power line and signal line. As the microphone cable, a two-core shield covering line in which the outer peripheral surface of shield covering line is covered with an external sheath (skin) is used. The output module section is sometimes called a power module section because it supplies power to the condenser microphone unit.
The part of microphone cable is vulnerable to noise (electromagnetic waves) from the outside because audio signals are transmitted imbalancedly. Therefore, strong electromagnetic waves that are applied to the microphone cable intrude into the condenser microphone unit or the output module section, and are detected by a semiconductor device etc., by which noise is sometimes generated.
In particular, a cellular phone, which has come into wide use in recent years, generates considerably strong electromagnetic waves (for example, in the range of several centimeters to several tens centimeters, field intensity reaching several ten thousands of intensity of field generated in the city by commercial electric waves), so that measures against electromagnetic waves generated from cellular phones are a pressing need in the field of microphone.
The flexible pipe functions as a shield for the microphone cable but does not function perfectly. Specifically, the flexible pipe is manufactured by forming a coil spring for carrying a restoring force using a round wire rod such as a steel wire rod and by putting a triangular wire rod, which has a triangular cross section and is plastically deformed, in a clearance between the coil springs from over the coil spring. Therefore, the flexible pipe has an impedance, though being a low resistance value (for example, about 1Ω), in the contact portion between these wire rods.
The condenser microphone unit and the output module section each have a reliable shied case. However, when viewed as the whole of microphone, the part of flexible cable is shielded imperfectly. Therefore, a high-frequency current due to strong electromagnetic waves intrudes into the microphone via the flexible pipe, and resultantly noise is sometimes generated as described above.